The detection scheme described in this specification is employed in a network analyzer of the heterodyne class. Heterodyning has been the principal signal processing technique used in both network and spectrum analyzers for many years.
Heterodyning refers to the process by which the modulation signals in a frequency band of interest are successively shifted lower in frequency by mixing and filtering operations. Typically, the final signal frequency band is centered at an intermediate frequency which is convenient for building detectors to extract envelope and phase information (or real and quadrature components) and display information in linear or log form. In the case of most spectrum analyzers, only the envelope (or modulus) is recovered.
First generation spectrum analyzers use completely analog signal processing. That is, all signals, including the measures of magnitude and phase (or real and quadrature components) are represented by, and processed in, analog voltages. These are then conditioned for display on a cathode ray tube or plotter.
A second generation of analyzers use digital conversion of the signals, which conversion occurs after the heterodyning process and detection of the signal at the intermediate frequency. The detector analog signal output, representing magnitude and phase of the modulation signal of interest, is then coverted to digital data for ease of storage and display of swept frequency data. Conversion to digital data facilitates averaging and other post detection processing which is difficult to do in entirely analog circuitry.
Referring to FIG. 1, prior art detection schemes produce magnitude and phase samples. The signal processing of this type of detector is very critical, and such a detector is correspondingly expensive to produce. The center frequency and bandwidth are determined by an analog filter which must be compensated for drift, and separate circuitry is required to produce phase information.
The present invention is incorporated into a network analyzer which represents the first of another generation of analyzers. Such analyzers can be characterized by the hybrid nature of the signal processing, namely, converson of the signals to digital form during the heterodyning process. In the detector of the present invention, two heterodyne frequency shifts precede analog-to-digital conversion. The signal sampling, part of the conversion process, produces another shift, while the final shift is entirely digital.
The center frequency, bandwidth and phase reference of the detector of the present invention are essentially drift-free because these parameters are synthesized from a common reference. Since the analog-to-digital converter (ADC) in the detector is common to both in-phase and quadrature parts of the signal, the amplification and orthogonality of the parts is essentially matched. By properly selecting the sample rate and local oscillator phase, the mixers do not actually multiply, which in turn reduces circuitry and simplifies implementation.